1. Field of the Invention
The invention is related to semiconductor materials, methods, and devices, and more particularly, to the growth of and reactor components for the growth of group III-nitride materials by hydride vapor phase epitaxy (HYPE).
2. Prior Art
This application references a number of patents, applications and/or publications. Each of these patents, applications and/or publications is incorporated by reference herein.
The usefulness of gallium nitride (GaN), aluminum nitride, indium nitride, and their ternary and quaternary compounds (AlGaN, InGaN, AlInGaN), collectively known as “group III-nitrides,” has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices (see T. Nishida and N. Kobayashi, Phys. Stat. Sol. (a), 188 (1), 113 (2001); S. Nakamura, G. Fasol, and S. J. Pearton, The Blue Laser Diode. New York: Springer, 2000; and L. F. Eastman and U. K. Mishra, IEEE Spectrum, 39 (5), 28 (2002)). These devices are typically grown epitaxially by growth techniques including molecular beam epitaxy (MBE) (see S. Yoshida, S. Misawa and S. Gonda, Appl. Phys. Lett. 42 (1983), pp. 427), metalorganic chemical vapor deposition (MOCVD) (see H. M. Manasevit, F. M. Erdmann and W. I. Simpson, J. Electrochem. Soc. 118 (1971), pp. 1864), or hydride vapor phase epitaxy (HVPE) (see H. P. Maruska and J. J. Tietjin, Appl. Phys. Lett. 15 (1969), pp. 327). Among these three techniques, HVPE has the advantage of a high growth rate, which is more than a factor of ten higher than those inherent to MOCVD or MBE, making HVPE most preferable for the growth of thick III-Nitride films, templates, free-standing substrates, and bulk crystals.
Group III-nitrides are most stable in the hexagonal würtzite crystal structure, in which the crystal is described by two (or three) equivalent basal plane axes that are rotated 120° with respect to each other (the a-axes), all of which are perpendicular to a unique c-axis. FIG. 1 is a schematic of a generic non-primitive hexagonal würtzite crystal structure 101 and with the a1, a2, a3, and c axes identified therein as blocks 102, 103, 104, and 105, respectively. Using common crystallographic notation, the four principal axes in the wOrtzite structure are parallel to the [2 110], [ 12 10], [ 1120], and [0001] directions, respectively.
FIG. 2 illustrates the atomic lattice arrangement of a würtzite group III-nitride crystal. As a consequence of the group III and nitrogen atom positions within the würtzite structure, as one proceeds from plane to plane along the c-axis, each plane will contain only one type of atoms, either group III atoms or N atoms only. In order to maintain charge neutrality, group III-nitride crystals terminate with one c-face that contains only nitrogen atoms (the N-face, block 202), and one c-face that only contains group III atoms (the Ga-face, Al-face, In-face, or group III-face, block 201). Using common Miller-Bravais {hkil} index notation, the polar c-planes can be collectively described by the indices {0001}. More specifically, the metal-terminated c-plane is referred to as the (0001) plane while the nitrogen-terminate c-plane is referred to as the (000 1) plane. Alternate notation for these planes that recognizes the redundancy of the h, k, and i indices is (00.1) and (00. 1) for the metal-terminated and N-terminated c-planes, respectively. As a consequence, group III-nitride crystals are polarized along the c-axis. The spontaneous polarization of these crystals is a bulk property and depends on the structure and composition of the crystal.
The second type of polarization in group III-nitrides is piezoelectric polarization. Piezoelectric polarization occurs when the group III-nitride material experiences a compressive or tensile strain, as can occur when (Al, In, Ga, B)N layers of dissimilar composition (and therefore different lattice constants) are grown in a nitride heterostructure. For example, a thin AlGaN layer on a GaN template will have in-plane tensile strain, and a thin InGaN layer on a GaN template will have in-plane compressive strain, both due to lattice mismatch between the dissimilar composition layers. For an InGaN quantum well grown on a GaN template, the piezoelectric polarization will point in the opposite direction than that of the spontaneous polarization of the InGaN and GaN. For an AlGaN layer grown upon GaN, the piezoelectric polarization will point in the same direction as that of the spontaneous polarization of the AlGaN and GaN. Note that the chemical formula “AlGaN” as used herein refers to any III-nitride composition consisting predominantly of aluminum, gallium, and nitrogen, generally described by the formula AlxGa1-xN in which 0<x<1.
One possible approach to eliminating the spontaneous and piezoelectric polarization effects in GaN optoelectronic devices is to grow the devices on non-polar planes of the crystal (see P. Waltereit, O. Brandt, A. Trampert, H. T. Grahn, J. Menniger, M. Ramsteiner, M. Reiche, and K. H. Ploog, Nature (London), 406, 865 (2000); M. D. Craven, P. Waltereit, F. Wu, J. S. Speck, and S. P. DenBaars, Jpn. J. Appl/Phys 42 (3A) 235 (2003); and H. M. Ng, Appl. Phys. Lett., 80 (23) 4369 (2002)). Generally speaking, any crystal plane in the würtzite III-nitride crystal system having indices {hki0}/{hk.0} can be described as a non-polar plane. The two most common non-polar plane orientations are the {11.0} a-planes and {10.0} m-planes (block 203 in FIG. 2). Such planes contain equal numbers of group III and N atoms and are charge-neutral. Furthermore, subsequent non-polar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction. Growth on electronic devices, such as high electron mobility transistors; or optoelectronic devices, such as visible and ultraviolet laser diodes and light-emitting diodes; in m-plane orientations could yield significantly enhanced device performance compared to equivalent devices grown on c-plane III-nitride planes.
Semi-polar planes represent a third class of crystal orientations that are relevant for the fabrication of III-nitrides, the growth of which were described by Baker et al in U.S. Pat. No. 7,220,324. A semi-polar plane is any crystal plane having at least two non-zero h, k, and/or i Miller-Bravais indices and a non-zero I Miller-Bravais index. Some common examples of semi-polar planes are the {30.1}, {20.1}, {10.1} (block 204 in FIG. 2), {10.2}, {10.3}, and {11.2} (block 205 in FIG. 2) families of planes. The nitride crystal's polarization vector lies neither within such planes or normal to such planes, but rather lies at some angle inclined relative to the plane's surface normal. For example, the {10.1} and {10.3} planes are at 62.98° and 32.06° to the (00.1) c-plane, respectively.
Semi-polar plane group III-nitride devices provide advantageous electronic and optoelectronic properties over conventional c-plane group III-nitride devices by virtue of a reduction in net polarization in the conduction direction of the devices for certain compositions. As described in A. E. Romanov, T. J. Baker, S. Nakamura, J. S. Speck, J. Appl. Phys. 100, 023522 (2006), the piezoelectric polarization can offset some or all of the spontaneous polarization for certain combinations of InGaN quantum wells on GaN templates/barriers on particular semi-polar crystal planes. Semi-polar III-nitride devices can therefore offer electrical characteristics that approach those of non-polar III-nitride devices but with unique and oftentimes favorable epitaxy characteristics.
Bulk crystals of single-crystal group III-nitride materials are not readily manufactured so it is impractical or impossible to simply cut a crystal to present a surface for subsequent device regrowth. Commonly, group III-nitride films are initially grown heteroepitaxially, i.e. on foreign substrates that provide a reasonable lattice match to the desired III-nitride material. The heteroepitaxial growth process is commonly carried out with MOCVD, MBE, or HVPE; most typically on substrates consisting of sapphire, silicon carbide, or silicon. Other substrate materials of practical use for group III-nitride heteroepitaxy include, but are not limited to, lithium aluminate, and magnesium aluminate spinel.
The overwhelming majority of III-nitride heteroepitaxy performed today is based on c-plane GaN growth on c-plane sapphire, c-plane silicon carbide, or (111) silicon substrates. Such c-plane GaN growth is imperfect but well-established within the visible optoelectronics and power electronics industries.
High quality GaN films having smooth, mirror-like surfaces have been obtained by MOCVD by incorporating a thin buffer layer into the structure. However, MOCVD is not appropriate for growing thick GaN or other group III-nitride films due to its low growth rate in the order of 1 μm/hour. Due to this reason, HVPE has been utilized in GaN film growth on sapphire substrates in spite of the resulting GaN films typically having a rough surface and poor crystalline characteristics. Attempts to improve the HVPE-grown GaN material quality have utilized aluminum nitride (AlN), zinc oxide (ZnO) or low-temperature GaN films as an interface buffer layer between the sapphire and GaN, similar to buffer layer methodologies developed for MOCVD growth of GaN. However, little distinct quality improvement from the buffer layers has been observed in the resulting GaN films (see H. Amano, N. Sawaki, and Y. Toyada, Appl. Phys. Lett. 48 (1986), pp. 353; S. Nakamura, Jpn. J. Appl. Phys. 30 (1991), pp. 1705; and T. Detchprohm, H. Amano, K. Hiramatsu and I. Akasaki, J. Cryst. Growth 128 (1993), pp. 384).
The problem of poor crystalline quality in heteroepitaxially grown group III-nitrides is particularly acute for semi-polar III-nitrides, particularly those for which multiple crystal orientations can readily nucleate on the same substrate (see T. J. Baker, B. A. Haskell, F. Wu, J. S. Speck, and S. Nakamura, Jpn. J. Appl. Phys 45 (6) L154 (2006)). For example, GaN films grown on m-plane sapphire can orient themselves on three different planes, the {1 100}, {10.3}, and the {11.2} planes. Commonly multiple domains of different orientations or rotational senses of these various GaN planes will appear in a single heteroepitaxial film on an m-plane sapphire substrate. For such heteroepitaxially grown III-nitride films to be useful for device fabrication, a means is required to ensure single phase/orientation growth of the III-nitride on the foreign substrate. It is therefore desirable to develop a superior method for the growth of high-quality thick (e.g. 2-100,000 μm) group III-nitride films based on HVPE.
FIG. 3 provides a schematic of a generic HVPE growth chamber 300 as has been described in the prior art. Block 301 depicts a containment vessel, commonly a quartz tube with appropriate end seals to prevent interaction of process gases with the outside atmosphere. Block 302 depicts one or more substrates placed on a susceptor 303, which is located within a heated growth zone of the chamber (304). Adjacent to the growth zone is the source zone of the chamber 305, in which one or more metal supply tubes or vessels 306 charged with metals 307 are located. The metal supply tubes 306 are connected to gas supply lines that are typically isolated from the gas sources 309 by shutoff valves 308. Typical gases that are supplied to the metal sources include HCl and carrier gases (CG) that include, but are not limited to, argon, nitrogen, helium, and hydrogen. The transportation agents of elements of Group V are hydrides (i.e., ammonia) which are brought into the reactor by a separate gas line. The vapor phase (GaCl+NH3) is transported from the source zone towards the growth zone of the reactor as depicted by block 310, where the substrate(s) 302 is/are located, by the carrier gas and process gas. The waste process gas exits the chamber through the process exhaust 311.
The basic chemistry of the group III-nitride HVPE process that occurs in a reactor 300 is described by the expressions in Equations 1-7:HCl(g)+Ga(l)→GaGl(g)  [1]HCl(g)+Al(s/l)→AlGl(g)  [2]HCl(g)+In(s/l)→InCl(g)  [3]NH3+GaCl(g)→GaN(s)+HCl(g)+H2(g)  [4]NH3+AlCl(g)→AlN(s)+HCl(g)+H2(g)  [5]NH3+InCl(g)→InN(s)+HCl(g)+H2(g)  [6]NH3+xGaCl(g)+yAlCl(g)+zInCl(g)→(GaxAlyInz)N(s)+HCl(g)+H2(g)  [7]
Equations 1-3 describe the formation of gaseous group III monochlorides by flowing hot HCl gas over a group III metal 307 in a metal supply tube or vessel 306. Depending on the temperature at which the formation reactions occur, a metal-trichloride may form as an intermediate product instead of the metal-monochloride. However, such trichlorides typically dissociate at typical group III-nitride HVPE growth temperatures to yield the monochloride that is most active in HVPE growth. The formation reaction typically occurs by flowing HCl gas over a group III source consisting of gallium, aluminum, and/or indium metal, at a temperature ranging from approximately 300° C. to approximately 1100° C. In an appropriately designed metal supply vessel 306 with optimized HCl and carrier gas flows, nearly 100% of the input HCl will react with exposed metal to form the metal monochloride. It should be noted that other halide compounds, including but not limited to hydrogen bromide and hydrogen iodide, are occasionally substituted for hydrogen chloride in the HVPE chemistry. Such substitutions do not fundamentally alter the HVPE growth process from that described herein.
Equations [4-6] describe the growth of simple group III-nitrides in the HVPE process through reaction of group III-monochlorides and ammonia, a group V hydride. Similarly, Equation [7] describes growth of a group III-nitride which can occur over a wide range of temperatures, and is most rapid when the precursors converge at a surface within the HVPE growth chamber. The preferred surface for the reaction to take place is a substrate as depicted by block 302 in FIG. 3. However, parasitic, or unproductive, deposition of group III-nitrides can occur on virtually any surface with an HVPE growth chamber.
In the method of hydride vapor-phase epitaxy that is often used as a technique of quick growth of GaN and other III-nitrides, the reactors have hot walls as would be provided by the external heaters 304 and 305 to assure the stability of transportation agents of elements of Group III, in the form of GaCI, obtained by reacting gaseous HCl with liquid gallium inside the reactor. For example, in U.S. Pat. No. 6,632,725, adding into the carrier gas a quantity of hydrochloric acid, at a total constant flow under classical negative supersaturation conditions, enables a high growth rate of the epitaxial layer while decreasing and even avoiding the parasitic GaN nucleation or deposition on the walls of the reactor.
In U.S. Pat. No. 7,621,999, AlxGa1-xN is grown according to the HVPE process in which use is made of an aluminum source, a gallium source, an ammonia source and a carrier gas. In U.S. Pat. No. 6,528,394, a gas mixture of ammonia (NH3) and HCl is used as a treatment for sapphire substrates. This treating of the sapphire substrate by flowing a gas mixture of ammonia and hydrogen chloride onto the substrate was used for modification of substrate surface—not for growing the group III-nitride, because this mixture did not contain any metal chlorides.
US Patent Application Publication No. 2012/0156863 describes exposing a surface of the aluminum oxide containing substrate to a pretreatment gas mixture, wherein the pretreatment gas mixture comprises ammonia and a halogen gas comprising chlorine (Cl2) gas. They use a treatment procedure of treating the substrate by using halogen gas.
While many examples of substrate pretreatment are described in the prior art, none have proven effective for establishing orientation control and enhancing structural quality of semi-polar group III-nitride heteroepitaxially grown films and materials.